Jurassic Plant Fossils from Cuba Evidence of a· Laurasian Coastal Forest?

T oda y we know that the greatest plant diver­sity on the planet is concentrated in the tropics, and Cuba, the largest island in the

Caribbean, has the richest vegetation and highest proportion of endernisrn of the entire region. About half of its approxirnately 6,000 species of flower­ing plants and approxirnately 70 genera are unique to the island.

Above: Life restoration ofthe enigmatic San Cayetano Forrnation ofwestem Cuba. Elements indude the bivalve Trigonia kroemmelbeini, decapad crustaceans, small perisphinctids ammonites, abundant ferns (Pteridophita: Piazopteris branneri), trees (resembling Araucarioxylon), and small water-loving ferns resemblingConiopteris. ©Heraldo Mussolini; used by perrnission.

Possil News - Summer 2016

One example ofCuba's rernarkable modern-day plant life is the so-called "cork palrn," Microcycas calocoma, endernic to a srnall area in Pinar del Río Province on Cuba's westernrnost tip. Microcycas, descended frorn a prirnitive lineage of Mesozoic plants (roughly 230 to 65 rnillionyears ago), is con­sidered a ''living fossil."

Once the supercontinent of Pangaea had broken apart, the tropical coasts of Laurasia and Gondwa­na opened onto a proto-Caribbean seaway. A vari­ety of studies ha ve concluded that the tropical coasts along this ancient sea were populated by a high di­versity ofterrestrial plant life, as is dernonstrated by the relatively abundant fossil rernains found in Ju­rassic outcrops in Pinar del Río. These fossils have considerable value in interpreting the clirnate and environrnent of prehistoric times.

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Petrified wood from the Jagua Formation.

MA-1c.in~ th.e f{u'1c.s 5J'eA-1c. Geologists assigned rocks found in western Cuba to the Upper Jurassic (early Oxfordian) period and named them the San Cayetano Formation because of their proximity to the town of San Cayetano, in the northeast Vueltabajo area. Thousands offern im­pressions have been discovered in the San Cayetano Formation, one of Cuba's most enigmatic.

In 1965, the Russian paleontologist V.A. Vachrameev identified fragments of sterile and fertile fronds of a pteridophyte plant as a new species, Phlebopteris cubensis, and la ter published the first scientific study of J urassic plant fossils in Cuba. In Vachrameev's view, Phlebopteris grew in a swamp or lake near the shoreline, and he considered the species related to varieties found in Eurasia and Greenland.

Sorne years later, however, Cuban paleontologist Alberto Areces-Mallea revised the taxon after study­ing better preserved material collected between 1984 and 1988. He suggested that Phlebopteris cubensis was actually a synonym of Piazopteris branneri, a species known since 1914 from the Ear­ly Jurassic Huayacocotla Formation ofMexico. This same fern has ALSO been reported from J urassic rocks in Honduras and northern Africa.

Areces-Malle a not only proposed a correlation between the age ofthe San Cayetano Formation and the Huayacocotla Formation, he determined that the paleoflora ofboth formations was very similar,

32

indicating that the Early Jurassic climate was warm and humid across the en tire Gulf ofMexico. He further hypothesized that the outcrops containing Piazopteris branneri fossils represented the ancient coastal delta of a large river that drained from the con­tinent to the south of Cuba, where the Yucatán basin exists today.

A- Win~ulil un th.e '?Mt ••• 5titt r>nt"t ~A-lf~r>!'en The San Cayetano

Formation produced another remarkable discovery in 1976 when Polish geologist Grzegorz Haczewski reported accumulations of plant debris, poorly pre­served pollen and spores, and petrified wood resembling Araucarioxylon, an extinct genus of co­nifer known only from Triassic deposits in North America and, therefore, presumably a more ancient plant than Piazopteris. In a letter, Haczewski re­marked that "the growth-rings in the fossils were slightly marked and carbonization [ was present] that suggested fire."

Deepening the mystery, in February1999, a num­ber of fossil specimens of another strange and very tiny plant were discovered in the San Cayetano For­mation near the town of Matahambre during intensive field work by paleontologists Manuel Itur­ralde-Vinent and Reinaldo Rojas-Consuegra ofthe Havana Museum ofNatural History.

These rare specimens were conserved in the museum's paleontology collections and tentatively identified by experts at the Université Pierre-et­Marie-Curie in París as a small fern from the Family Hymenophyllaceae.

In September 2011, however, paleobotanist Sid­ney R. Ash from the University of New Mexico examined photographs ofthe specimens and argued that they might, instead, be examples of the Juras­sic fern, Coniopteris. "These fossils very closely resemble sorne sterile forms of Coniopteris simplex, a member of the Family Dicksoniaceae," Ash wrote, "but without seeing the sporangia [i.e., the struc-

Fossil News - Summer 2016

tures in which spores form and are enclosed] it just isn't possible to determine with total confi­dence which family the fossils belong to. I would think that these ferns, whatever they are called, lived in a humid environment along rivers or a coastline and were transported only a short dis­tance prior to burial on a coastal plain."

Based upon the same photographs, another detective of the past, the paleobotanist Brian Ax­smith, concurred with the identification of the specimens as Coniopteris. "Coniopteriswas very common during the J urassic period," he re­marked, "so itwouldn't surprise meto learn they were discovered [in that part of the Caribbean]."

A-!Juun,e.r f'!A-nt-~e.A-rin' :ffvrizcm Other well-preserved plant fossils, including fine leaf detritus, fern impressions, logs, large trunks, and branch and wood fragments (sorne ofthem showing bore marks) were collected by Manuel Iturralde-Vinent and his colleagues at the Ha­vana Museum ofNatural Historywhile theywere searching for marine reptiles, pterosaurs, and di­nosaurs in the fossiliferous mid-to early-Late Oxfordian J agua Formation in the Sierra de Los Órganos, a mountain range in the Guaniguani­co Cordillera in Pinar del Río province. These beds also yield ammonites, bivalves, and fish.

Iturralde-Vinent's expedition yielded a large collection of plant fossils whose identification has yet to be completed. In the meantime, they are housed at the Havana Museum ofNatural Histo­ry, awaiting future research.

Borings in fossil wood produced by marine bivalves.

Iturralde-Vinent's fossils, however, are believed to have come from plants that lived in an ancient near-shore forest or "savannah" that was also home

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to large herbivorous sauropod dinosaurs, including Camara­saurids, sorne 156 million years ago. The vegetation may have provided an excellent food source for these dinosaurs.

From all evidence, however, dinosaurs were not the only creatures to have nourished themselves on the abundant plant life of this ancient coast-al forest. More recently, tiny ichnofossils (trace fossils) found in petrified wood that I turralde-Vinent collected from the J agua Formation were interpreted as bore holes, presumably produced by ma­rine bivalves (probably pholadids of the subfamily Martesiinae) that colonized floating logs. These fossil traces were given the name Teredolites clavatus. The logs on which Teredolites lived and fed were likely transported to the sea by rivers that flowed from an ex­posed land mass, possibly on the supercontinent of Laurasia.

The discovery and study of plant fossils from formations in

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The Rudest Bivalve

western Cuba add depth and di­mension to our picture of what life was like in this part of the planet during the time ofthe di­nosaurs and can tell us how Cuba's ancient vegetation was similar to or different from plants that grew elsewhere. Much ofthe rich variety of plant fossils in Cu­ba's Jurassic rocks, however, has

yet to receive thorough study. Until that happens, we must wait to know the full story they might tell.

- Yasmani Ceballos Izquierdo; English translation by Yasmani Ceballos Izquierdo and Wendell Ricketts

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